The hardest decision of my life is to continue to live on this planet. Sorry if that sounded a bit (or a lot) cheesy, but that's the truth. It's not fate we are suffering/existing/living, it's our choice. There were many instances in life where I had to make a difficult decision, even when it was simple. We always have a choice, even if all the remaining options are bad ones. This is the only thing which comes to my mind when I think of the hardest decision I had to make. Well not counting those in RPGs of course.